Process and apparatus for a wavelength tuning source

ABSTRACT

An apparatus and source arrangement for filtering an electromagnetic radiation can be provided which may include at least one spectral separating arrangement configured to physically separate one or more components of the electromagnetic radiation based on a frequency of the electromagnetic radiation. The apparatus and source arrangement may also have at least one continuously rotating optical arrangement which is configured to receive at least one signal that is associated with the one or more components. Further, the apparatus and source arrangement can include at least one beam selecting arrangement configured to receive the signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 11/867,953 filed Apr. 11, 2008, now U.S. Pat. No. 7,724,786 which isa divisional of U.S. patent application Ser. No. 10/861,179 filed Jun.4, 2004, which issued as U.S. Pat. No. 7,519,096 on Apr. 14, 2009. Thisapplication also claims priority from U.S. patent application Ser. No.60/476,600 filed on Jun. 6, 2003, and U.S. patent application Ser. No.60/514,769 filed on Oct. 27, 2003, the entire disclosure of which isincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The invention was made with the U.S. Government support under GrantNumber DAMD17-99-2-9001 awarded by the U.S. Department of the Army andGrant Number BES-0086789 awarded by the National Science Foundation.Thus, the U.S. Government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to optical systems and moreparticularly to an optical wavelength filter system for wavelengthtuning.

BACKGROUND OF THE INVENTION

Considerable effort has been devoted for developing rapidly and widelytunable wavelength laser sources for optical reflectometry, biomedicalimaging, sensor interrogation, and tests and measurements. A narrow linewidth, wide-range and rapid tuning have been obtained by the use of anintra-cavity narrow band wavelength scanning filter. Mode-hopping-free,single-frequency operation has been demonstrated in an extended-cavitysemiconductor laser by using a diffraction grating filter design.Obtaining single-frequency laser operation and ensuring mode-hop-freetuning, however, may use a complicated mechanical apparatus and limitthe maximum tuning speed. One of the fastest tuning speeds demonstratedso far has been limited less than 100 nm/s. In certain applications suchas biomedical imaging, multiple-longitudinal mode operation,corresponding to an instantaneous line width as large or great than 10GHz, may be sufficient. Such width may provide a ranging depth of a fewmillimeters in tissues in optical coherence tomography and amicrometer-level transverse resolution in spectrally-encoded confocalmicroscopy.

A line width on the order of 10 GHz is readily achievable with the useof an intra-cavity tuning element (such as an acousto-optic filter,Fabry-Perot filter, and galvanometer-driven diffraction grating filter).However, the sweep frequency previously demonstrated has been less than1 kHz limited by finite tuning speeds of the filters. Higher-speedtuning with a repetition rate greater than 15 kHz may be needed forvideo-rate (>30 frames/s), high-resolution optical imaging in biomedicalapplications.

Accordingly, there is a need to overcome the above-describeddeficiencies.

SUMMARY OF THE INVENTION

According to the exemplary concepts of the present invention, an opticalwavelength filter may be provided that can be tuned with a repetitionrate of greater than 15 kHz over a wide spectral range. In addition, awavelength tuning source comprising such optical filter in combinationwith a laser gain medium may be provided. The tuning source may beuseful in video-rate optical imaging applications, such as the opticalcoherence tomography and spectrally encoded confocal microscope.

In general, the optical filter according to one exemplary embodiment ofthe present invention may include a diffraction grating, a rotatingpolygon scanner, and a telescope. Such optical filter can be operated ata tuning speed more than an order of magnitude higher than theconventional filters. The wavelength tunable light source may beimplemented by employing the filter, e.g., in combination with a lasergain medium. The filter and gain medium may further, be incorporatedinto a laser cavity. For example, a laser can emit a narrow bandspectrum with its center wavelength being swept over a broad wavelengthrange at a high repetition rate.

In one exemplary embodiment of the present invention, an apparatus isprovided which includes an arrangement for emitting an electromagneticradiation that has a spectrum whose mean frequency changes substantiallycontinuously over time. Such radiation is may be associated with atuning speed that is greater than 100 terahertz per millisecond. Themean frequency can change repeatedly at a repetition rate that isgreater than 5 kilohertz or over a range greater than 10 terahertz. Thespectrum may have a tuning range covering a portion of the visible,near-infrared or infrared wavelengths. Exemplary spectra may be centeredat approximately at 850 nm, 1300 nm or 1700 nm wavelengths. Further, thespectrum may have an instantaneous line width that is smaller than 100gigahertz. The apparatus may also include a laser cavity with aroundtrip length shorter than 5 m. The apparatus may also have a polygonscanner arrangement which may be adapted to receive at least a portionof the emitted electromagnetic radiation and reflect or deflect theportion to a further location. In addition, a beam separatingarrangement can be provided which selectively receives components of theelectromagnetic radiation.

According to another exemplary embodiment of the present invention theapparatus for filtering an electromagnetic radiation can include atleast one spectral separating arrangement configured to physicallyseparate one or more components of the electromagnetic radiation basedon a frequency of the electromagnetic radiation. The apparatus may alsohave at least one continuously rotating optical arrangement that isconfigured to receive the physically separated components andselectively direct individual components to a beam selectingarrangement.

In one exemplary variation of the present invention, the spectralseparating arrangement includes a diffraction grating, a prism, a grism,an acousto-optic beam deflector, a virtual phased array, and/or anarrayed waveguide grating. The continuously rotating optical arrangementmay be a polygon mirror, a diffractive element, a substantially opaquedisk having an array of substantially transparent regions, and/or asubstantially transparent disk having an array of substantiallyreflective regions. The spectral separating arrangement may also includea holographic grating mounted on a substrate comprising a continuouslyrotating optical arrangement.

In another exemplary variation of the present invention the beamselecting arrangement may be an optical fiber, an optical waveguide, apinhole aperture, a combination of a lens with an optical fiber,waveguide or pinhole, and/or a spatial filter. The beam selectingarrangement can include a plurality of beam selecting elements, and theelectromagnetic radiation which is transmitted by the plurality of beamselecting elements may be combined. The signal may be reflected multipletimes from the continuously rotating optical arrangement before beingreceived by the selecting arrangement.

According to yet another exemplary embodiment of the present inventionthe apparatus for filtering an electromagnetic radiation may include atleast one spectral separating arrangement configured to angularlyseparate one or more components of the electromagnetic radiation basedon a frequency of the electromagnetic radiation. Such arrangement canalso include at least one angularly deflecting optical arrangement thatincludes a pivot point, and that is configured to receive the componentsof the electromagnetic radiation and selectively direct the componentsto a beam selecting arrangement. Further, the arrangement can include atleast one optical imaging arrangement configured to receive thecomponents of the electromagnetic radiation and generate an image of oneor more dispersive elements associated with the components. The positionof the pivot point of the angularly deflecting optical arrangement maybe provided in proximity to a real or virtual image of at least one ofthe dispersive elements.

In one exemplary variant of the present invention, a deflection point ofthe angularly deflecting optical element may substantially overlap witha real image of at least one of the dispersive elements. At least onereflector which is configured to receive at least one signal from the atleast one angularly deflecting optical arrangement may also be provided.One or more of the dispersive elements may be a diffraction grating, aprism, a grism, an acousto-optic beam deflector, a virtual phased array,and/or an arrayed waveguide grating. The angularly deflecting opticalelement may be a polygon mirror scanner, a galvanometer mirror scanner,or a piezo-electric minor scanner.

According to still another exemplary embodiment of the presentinvention, an apparatus is provided for filtering an electromagneticradiation. The apparatus includes at least one dispersive arrangementconfigured to angularly separate components of the electromagneticradiation based on a frequency of the electromagnetic radiation, andgenerate frequency-separated components. The apparatus may also includeat least one angularly deflecting optical element having a pivot pointof an angular deflection. The pivot point can substantially overlap alocation where substantially all of the frequency-separated componentsoverlap.

In another exemplary embodiment of the present invention, at least onespectral separating arrangement can be provided that is configured tophysically separate one or more components of the electromagneticradiation based on a frequency of the electromagnetic radiation. Inaddition, at least one continuously rotating optical arrangement may beincluded which is configured to receive at least one signal that isassociated with the one or more components. At least one beam selectingarrangement may also be configured to receive the signal. The emittercan be a laser gain medium, a semiconductor optical amplifier, a laserdiode, a super-luminescent diode, a doped optical fiber, a doped lasercrystal, a doped laser glass, and/or a laser dye.

In still another exemplary embodiment of the present invention, a sourcearrangement can provide an electromagnetic radiation. The sourceincludes at least one emitter of the electromagnetic radiation, at leastone spectral separating arrangement configured to angularly separate oneor more components of the electromagnetic radiation based on a frequencyof the electromagnetic radiation, as well as at least one angularlydeflecting optical arrangement that includes a pivot point, andconfigured to receive the components of the electromagnetic radiation togenerate at least one signal associated with the one or more components.In addition, the source arrangement can include at least one beamselecting arrangement adapted to receive the signal, and selectivelygenerate at least one selected signal, and at least one optical imagingarrangement configured to received the selected signal, and generate animage of one or more dispersive elements associated with the one or morecomponents. In a variation of the present invention, more than one lasergain medium providing electromagnetic radiation and at least onespectral separating arrangement configured to physically separate one ormore components of the electromagnetic radiation based on a frequency ofthe electromagnetic radiation can be provided. In this variation, theselected components of electromagnetic radiation from each laser gainmedium are synchronized, and can be used separately or combined.

In one further exemplary embodiment of the present invention, ahigh-speed tuning of an extended-cavity semiconductor laser may beprovided. The laser resonator may include a unidirectional fiber-opticring, a semiconductor optical amplifier as the gain medium, and ascanning filter based on a polygon scanner. Variable tuning rates of upto 1,150 nm/ms (15.7 kHz repetition frequency) can be obtained over a 70nm wavelength span centered at 1.32 μm. Such tuning rate can be morethan an order of magnitude faster than is conventionally know, and maybe facilitated in part by self-frequency shifting in the semiconductoroptical amplifier. The instantaneous line width of the source may be<0.1 nm for 9-mW cw output power, and a low spontaneous-emissionbackground of 80 dB can be obtained.

Other features and advantages of the present invention will becomeapparent upon reading the following detailed description of embodimentsof the invention, when taken in conjunction with the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the invention will becomeapparent from the following detailed description taken in conjunctionwith the accompanying figures showing illustrative embodiments of theinvention, in which:

FIG. 1A is a block diagram of a first exemplary embodiment of an opticalwavelength filter according to the present invention;

FIG. 1B is a block diagram of a second exemplary embodiment of theoptical wavelength filter according to the present invention;

FIG. 1C is a block diagram of a third exemplary embodiment of theoptical wavelength filter according to the present invention;

FIG. 1D is a block diagram of a fourth exemplary embodiment of theoptical wavelength filter according to the present invention;

FIG. 1E is a block diagram of a fifth exemplary embodiment of theoptical wavelength filter according to the present invention;

FIG. 1F is a block diagram of a sixth exemplary embodiment of theoptical wavelength filter according to the present invention;

FIG. 1G is a block diagram of a seventh exemplary embodiment of theoptical wavelength filter according to the present invention;

FIG. 2 is a graph of exemplary characteristics of the optical wavelengthfilter according to the present invention;

FIG. 3 is an exemplary embodiment of the wavelength tuning laser sourceaccording to the present invention;

FIG. 4A is a graph of exemplary first output characteristics (laserspectrum vs. wavelength) of the laser source according to the presentinvention;

FIG. 4B is a graph of exemplary second output characteristics (outputpower vs. time) of the laser source according to the present invention;

FIG. 5 is a graph of exemplary output power provided as a function ofsweep speed according to the present invention;

FIG. 6 is an exemplary embodiment of a free-space extended-cavitysemiconductor tunable laser arrangement according to the presentinvention;

FIG. 7 is an illustration of a seventh exemplary embodiment of theoptical wavelength filter according to the present invention;

FIG. 8 is a schematic diagram of an exemplary embodiment of aspectrally-encoded confocal microscope that utilizes the tunable lasersource according to the present invention;

FIG. 9 is a schematic diagram of an exemplary embodiment of afrequency-domain optical coherence tomography arrangement that utilizesthe tunable laser source according to the present invention;

FIG. 10A is a top view of an eighth exemplary embodiment of thewavelength filter according to the present invention; and

FIG. 10B is a perspective plan view of the wavelength filter shown inFIG. 10A.

DETAILED DESCRIPTION

FIG. 1A shows a block diagram of a first exemplary embodiment of anoptical wavelength filter 1 in accordance the present invention. In thisfirst exemplary embodiment, the optical wavelength filter 1 can be usedin a variety of different applications, general examples of which aredescribed below. In this example, the filter 1 may be coupled to one ormore applications 3 via a light source 2. It should be understood thatin certain exemplary applications, the filter 1 can be used with orconnected to an application (e.g., one or more of the applications 3)via a device other than a light source (e.g. a passive or active opticalelement). In the first exemplary embodiment shown in FIG. 1A, a broadspectrum light source and/or controller 2 (hereinafter referred to as“light controller”), may be coupled to a wavelength dispersing element4. The light controller 2 can be further coupled to one or more of theapplications 3 that are adapted to perform one or more tasks with orfor, including but not limited to, optical imaging processes and opticalimaging systems, laser machining processes and systems, photolithographyand photolithographic systems, laser topography systems,telecommunications processes and systems, etc. The wavelength dispersingelement 4 can be coupled to a lens system 6, which is further coupled toa beam deflection device 8.

The light controller 2 can be one or more of various systems and/orarrangements that are configured to transmit a beam of light having abroad frequency (f) spectrum. In one exemplary embodiment, the beam oflight may be a collimated beam of light The beam of light can include aplurality of wavelengths λ . . . λn, within the visible light spectrum(e.g., red, blue, green). Similarly, the beam of light provided by thelight controller 2 can also include a plurality of wavelengths λ . . .λn that may be defined outside of the visible spectrum (e.g.,ultraviolet, near infrared or infrared). In one exemplary embodiment ofthe present invention, the light controller 2 can include aunidirectional light transmission ring, which shall be described infurther detail below in connection with FIG. 3 which shows an exemplaryembodiment of a wavelength tuning laser source. Further, in anotherexemplary embodiment of the present invention, the light controller 2can include a linear resonator system, which shall be described infurther detail below in connection with FIG. 6.

The wavelength dispersing element 4 of the optical wavelength filter 1can include one or more elements that are specifically adapted toreceive the beam of light from the light controller 2, and toconventionally separate the beam of light into a plurality ofwavelengths of light having a number of directions. The wavelengthdispersing element 4 is further operative to direct portions of lighthaving different wavelengths in equal angular directions ordisplacements with respect to an optical axis 38. In one exemplaryembodiment of the present invention, the wavelength dispersing element 4can include a light dispersion element, which may include but notlimited to, a reflection grating, a transmission grating, a prism, adiffraction grating, an acousto-optic diffraction cell or combinationsof one or more of these elements.

The lens system 6 of the optical wavelength filter 1 can include one ormore optical elements adapted to receive the separated wavelengths oflight from the wavelength dispersing element. Light at each wavelengthpropagates along a path which is at an angle with respect to the opticalaxis 38. The angle is determined by the wavelength dispersing element 4.Furthermore, the lens system 6 is adapted to direct or steer and/orfocus the wavelengths of light to a predetermined position located on abeam deflection device 8.

The beam deflection device 8 can be controlled to receive andselectively redirect one or more discrete wavelengths of light backalong the optical axis 38 through the lens system 6 to the wavelengthdispersing element 4 and back to the light controller 2. Thereafter, thelight controller 2 can selectively direct the received discretewavelengths of light to any one or more of the applications. The beamdeflecting device 8 can be provided in many different ways. For example,the beam deflecting device 8 can be provided from elements including,but not limited to, a polygonal minor, a planar mirror disposed on arotating shaft, a mirror disposed on a galvonmeter, or an acousto-opticmodulator.

FIG. 1B shows a schematic diagram of a second exemplary embodiment ofthe optical wavelength filter F. The exemplary optical wavelength filter1′ can be configured as a reflection-type filter which may havesubstantially identical input and output ports. An input/output opticalfiber 10 and a collimating lens 12 can provide an input from a lightcontroller 2′ (which may be substantially similar to the lightcontroller 2 described above with reference to FIG. 1A) to the opticalwavelength filter 1′. The optical wavelength filter 1′ includes adiffraction grating 16, optical telescoping elements 6′ (hereinafterreferred to as “telescope 6′” and may possibly be similar to the lenssystem 6 of FIG. 1A), and a polygon mirror scanner 24. The telescope 6′can include two lenses, e.g., first and second lenses 20, 22 with 4-fconfiguration.

In the second exemplary embodiment of the optical wavelength filter 1′shown in FIG. 1B, the telescope 6′ includes the first and second lenses20, 22, which are each substantially centered along the optical axis 38.The first lens 20 may be located at a first distance from the wavelengthdispensing element 4′ (e.g., diffraction grating 16), which canapproximately be equal to the focal length F1 of the first lens 20. Thesecond lens 22 may be located at a second distance from the first lens20, which can be approximately equal to the sum of the focal length F1of the first lens 20 and the focal length F2 of the second lens 22.Using such arrangement, the first lens 20 can receive one or morecollimated discrete wavelengths of light from the wavelength dispersingelement 4′, and can effectively perform a Fourier Transform on each oneof the collimated one or more discrete wavelengths of light to provideone or more approximately equal converging beams that are projected ontoan image plane IP.

The image plane IP is preferably located between the first lens 20 andthe second lens 22 and at a predetermined distance from the first lens20. According to one exemplary variation of the present invention, suchpredetermined distance may be defined by the focal length F1 of thefirst lens 20. After such one or more converging beams are propagatedthrough the image plane IP, these one or more converging beams formequal or corresponding one or more diverging beams that are received bythe second lens 22. The second lens 22 is adapted to receive thediverging beams and provide approximately an equal number of collimatedbeams having predetermined angular displacements with respect to theoptical axis 38. Thus, the second lens 22 can direct or steer thecollimated beams to predefined portions of the beam deflection device8′.

The telescope 6′ according to the second exemplary embodiment of thepresent invention is operative to provide one or more features asdescribed above, as well as to convert a diverging angular dispersionfrom the grating into converging angular dispersion after the secondlens 22. Such result may be advantageous for a proper operation of thefilter. In addition, the telescope 6′ may provide adjustable parameterswhich control the tuning range and linewidth and reduce the beam size atthe polygon mirror to avoid beam clipping. As is illustrated in theexemplary embodiment of FIG. 1B, a beam deflection device 6′ (e.g.,which may include a polygon mirror or arrangement 24) is adapted topreferably reflect back only the spectral component within a narrowpassband as a function of the angle of the front minor facet of thepolygon arrangement 24 with respect to the optic axis 38. The reflectednarrow band light is diffracted and received by the optical fiber 10.The orientation of the incident beam 30 with respect to the optic axisand a rotation direction 40 of the polygon arrangement 24 can be used todetermine the direction of wavelength tuning, e.g., a wavelength up(positive) scan or a wavelength down (negative) scan. The exemplaryarrangement shown in FIG. 1B can generate a positive wavelength sweep.It should be understood that although the polygon arrangement 24 isshown in FIG. 1B as having twelve facets, polygon arrangements whichhave fewer than twelve facets or greater than twelve facets can also beused. While generally not considering practical mechanical limits, basedupon conventional manufacturing techniques, a particular number offacets of the polygon arrangement 24 to use in any application maydepend on a desired scanning rate and a scanning range for a particularapplication.

Furthermore, the size of the polygon arrangement 24 may be selectedbased on preferences of a particular application, and preferably takinginto account certain factors including, but not limited to,manufacturability and weight of the polygon arrangement 24. It shouldalso be understood that lenses 20, 22 that have different focal lengthsmay be provided. For example, the lenses 20, 22 should be selected toprovide a focal point at approximately the center point 24a of thepolygon arrangement 24.

In one exemplary embodiment, a Gaussian beam 30 can be utilized with abroad optical spectrum incident to the grating from the fiber collimator12. The well-known grating equation is expressed as λ=p·(sin α+sin β)where λ is the optical wavelength, p is the grating pitch, and α and βare the incident and diffracted angles of the beam with respect to thenormal axis 42 of the grating, respectively. The center wavelength oftuning range of the filter may be defined by λ₀=p·(sin α+sin β₀) whereβ₀ is the angle between the optic axis 38 of the telescope and thegrating normal axis. FWHM bandwidth of the filter is defined by(δλ)_(FWHM)/λ₀=A·(p/m)cos α/W, where A=√{square root over (4ln2)}/π fordouble pass, m is the diffraction order, and W is 1/e²-width of theGaussian beam at the fiber collimator.

Tuning range of the filter may be limited by the finite numericalaperture of the first lens 20. The acceptance angle of the first lens 20without beam clipping may be defined by Δβ=(D₁−W cos β₀/cos α)/F₁, whereD₁ and F₁ are the diameter and focal length of the first lens 20. Suchformulation relates to the filter tuning range via Δλ=p cos β₀·Δβ. Oneof exemplary design parameters of the filter, originated from themultiple facet nature of the polygon mirror, is the free spectral range,which is described in the following. A spectral component afterpropagating through the first lens 20 and the second lens 22 may have abeam propagation axis at an angle β′ with respect to the optic axis 38,e.g., β′=−(β−β₀)·(F₁/F₂), where F₁ and F₂ are the focal lengths of thefirst lens 20 and the second lens 22, respectively. The polygonarrangement 24 may have a facet-to-facet polar angle given byθ=2π/N≈L/R, where L is the facet width, R is the radius of the polygonand N is the number of facets. If the range of β′ of incident spectrumis greater than the facet angle, i.e. Δβ′=Δβ·(F₁/F₂)>θ, the polygonarrangement 24 can retro-reflect more than one spectral component at agiven time. The spacing of the multiple spectral componentssimultaneously reflected, or the free spectral range, can be defined as(Δλ)_(FSR)=p cos β₀(F₁/F₂)·θ. In an exemplary intra-cavity scanningfilter application, the free spectral range of the filter should exceedthe spectral range of the gain medium in order to avoid multiplefrequency bands (in the case of an inhomogeneously broadened gainmedium) or limited tuning range (in the case of a homogeneouslybroadened gain medium).

The duty cycle of laser tuning by the filter can be, for example, 100%with no excess loss caused by beam clipping if two preferable conditionsare met as follows:

$W < {\frac{\cos\;{\alpha F}_{1}}{\cos\;{\beta F}_{2}}L\mspace{14mu}{and}\mspace{14mu} W} < {\frac{\cos\;\alpha}{\cos\;\beta_{0}}{( {F_{2} - S} ) \cdot \theta}}$

The first equation may be derived from a condition that the beam widthafter the second lens 22 should be smaller than the facet width. Thesecond equation can be derived from that the two beams at the lowest 32and highest wavelengths 34 of the tuning range, respectively, whichshould not overlap each other at the polygon arrangement 24. S inequation (1) denotes the distance between the second lens 22 and thefront mirror of the polygon arrangement 24.

It is possible to select the optical components with the followingparameters: W=2.4 mm, p= 1/1200 mm, α=1.2 rad, β₀=0.71 rad, m=1,D₁=D₂=25 mm, F₁=100 mm, F₂=45 mm, N=24, R=25 mm, L=6.54, S=5 mm, θ=0.26rad, λ₀=1320 nm. From the parameters, the theoretical FWHM bandwidth,tuning range and free spectral range of the filter could be calculated:(Δλ)_(FWHM)=0.09 nm, Δλ=126 nm and (Δλ)_(FSR)=74 nm. Both conditions inequation (1) may be satisfied with particular margins.

FIG. 1C shows a diagram of a third exemplary embodiment of thewavelength tunable filter arrangement for doubling the tuning speed withthe same polygon rotation speed according to the present invention. Inthis exemplary embodiment, the mirror surface of the polygon arrangement24 is placed substantially a distance F2 from lens 22, and the beam oflight is reflected with a non-zero angle (rather than directly beingreflected back to the telescope from the polygon arrangement's 24 mirrorfacet). The sweep angle of the reflected light from the polygonarrangement 24 is double the polygon arrangement's 24 rotation angle.When the incident angle difference 90 between λ₁ and λ_(N) with respectto the polygon arrangement 24 is approximately the same as thefacet-to-facet angle 92 of the polygon, e.g., angle θ, the sweep angle94 of the reflected light is 2θ for a rotation of the angle θ of thepolygon arrangement 24. By placing two reflectors 100, 102, whichpreferably direct the reflected beam of light from the polygonarrangement 24 back to the polygon arrangement 24, and to the telescope(e.g., similar to the telescope 6′ of FIG. 1B), with the angle θ betweeneach other, twice wavelength scans from λ₁ to λ_(N) are achieved for thepolygon rotation of the one facet-to-facet angle θ.

In FIG. 1D which shows a fourth exemplary embodiment of the presentinvention, the incident angle 90 difference between λ₁ and λ_(N) to thepolygon arrangement 24 is smaller than polygon facet-to facet angle 92,e.g., ϕ (=θ/K, where K>1). This can be achieved by reducing the gratingpitch and increasing the F2/F1 ratio. In this exemplary embodiment, thefilter tuning speed may be increased by factor of 2K without increasingeither the rotation speed of the polygon arrangement 24 or the number offacets of the polygon arrangement 24.

The filter tuning speed can be further increased by having the beam oflight reflected multiple times by the polygon arrangement 24. A fifthexemplary embodiment of the present invention, depicted in FIG. 1E, isan arrangement for increasing the tuning speed by factor of 4K, where Kis the ratio of angle 92 to angle 90 (K=θ/ϕ). The beam of light isreflected twice (e.g., four times round trip) by the polygon arrangement24, so that the sweep angle 94 of the reflected light becomes angle 4θ,and the tuning speed becomes 4K times faster. Such reflection can alsobe assisted with the reflection of surfaces 100, 102, 104, 106 and 108.This exemplary embodiment of the filter arrangement can be used tobroaden the free spectral range (“FSR”) of the filter. For example, ifone of the final reflectors 102 in the embodiment shown in FIG. 1E isremoved, the FSR of the filter may become twice broader. It is likelythat there is no tuning speed enhancement in such case. Similarly, it ispossible to retain only one final reflector 100 in FIG. 1E. The FSR inthis embodiment can become four times broader.

FIG. 1F shows a sixth exemplary embodiment of the present inventionwhich provides a polygon tuning filter accommodating two light inputsand outputs. For example, in order to support two or more inputs andoutputs of this filter, two or more sets of optical arrangements, eachrespective set including an input/output fiber 10, 10′, a collimatinglens 12, 12′, a diffraction grating 16, 16′, and a telescope, may sharethe same polygon arrangement 24. Because the scanning mirror of thepolygon arrangement 24 is structurally isotropic about the rotationaxis, certain optical arrangements that can deliver the beams of lightto the polygon arrangement 24 can be accommodated from any directions.Since both sets of optical arrangement in the embodiment of FIG. 1Futilize the same polygon scanner, their respective scanning opticaltransmission spectra are synchronized. It should be understood that theexemplary embodiment of FIG. 1F can be extended to include multiple(greater than 2) optical arrangements each having its own input andoutput optical channel.

One exemplary application of the above-described polygon tuning filteraccording to the sixth embodiment of the present invention may be a wideband wavelength scanning light source. In FIG. 1G which shows a seventhexemplary embodiment of the present invention, a first broadband lightsource 60 provides a light signal which may have a wavelength λ₁ toλ_(i), and a second broadband light source 600 provides another lightsignal having a wavelength λ_(i-j) to λ_(N). When the two opticalarrangements supporting the wavelengths λ₁ to λ_(i) and the wavelengthsλ_(i-j) to λ_(N), respectively, are synchronized to output approximatelythe same wavelength at the same instance, such exemplary arrangement maybecome a wide band wavelength scanning light source with linear scanrate from λ₁ to λ_(N). Since the FSR of the polygon scanning filter canbe adjusted to be 200 nm or wider without any optical performancedegradation, two or more broadband light sources with different centerwavelengths can be combined with this filter to provide linear scanninglight source over 200 nm tuning bandwidth. It should be understood thatthe embodiment of FIG. 1G can be extended to include multiple (e.g.,greater than 2) optical arrangements and multiple (e.g., greater than 2)broadband light sources.

The exemplary embodiment illustrated in FIG. 1G can also be configuredso that the wavelength tuning bands of each optical arrangement andbroadband light source are discontinuous. In such a configuration, thetuning bands can be swept in a continuous or discontinuous sequentialmanner or be swept simultaneously.

FIG. 2 shows an exemplary graph of measured characteristics of thefilter according to an exemplary embodiment of the present invention.The normalized reflection spectrum of the filter, e.g., a curve 48, maybe measured by using broadband amplifier spontaneous emission light froma semiconductor optical amplifier (SOA) and an optical spectrumanalyzer. The optical spectrum analyzer can obtain or record anormalized throughput (reflected) spectrum in peak-hold mode while thepolygon arrangement 24 spins at its maximum speed of 15.7 kHz. Themeasured tuning range may be 90 nm which is substantially smaller thanthe theoretical value of 126 nm. It is possible to have a discrepancywhich may be due to an aberration of the telescope 6′, primarily fieldcurvature, associated with relatively large angular divergence of thebeam from the grating. Such aberration can be corrected using optimizedlens designs well known in the art. A curve 46 shown in FIG. 2illustrates the throughput spectrum when the polygon arrangement isstatic at a particular position. The observed free spectral range is73.5 nm, in agreement with a theoretical calculation. The FWHM bandwidthof curve 46 was measured to be 0.12 nm. The discrepancy between themeasured FWHM and the theoretical limit of 0.09 nm is reasonableconsidering the aberration and imperfection of the optical elements.

FIG. 3 shows an exemplary embodiment of the wavelength tuning lasersource according to the present invention. For example, thepolygon-based filter can be incorporated into an extended-cavitysemiconductor laser via a Faraday circulator 50. Intra-cavity elementsmay be connected by single-mode optical fibers 10. The gain medium maybe a semiconductor optical amplifier 52 (e.g., SOA, Philips, CQF 882/e).Laser output 72 may be obtained via the 90% port of a fiber-optic fusedcoupler 70. Two polarization controllers 64, 62 can be used to align thepolarization states of the intra-cavity light to the axes of maximumefficiency of the grating 16, and of the maximum gain of the SOA 50. Acurrent source 54 may provide an injection current to the SOA 50. Thepolygon arrangement 24 may be driven and controlled by a motor driver97. To generate a sync signal useful for potential applications,approximately 5% of the laser output may be directed to a photodetector82 through a variable wavelength filter 80 with bandwidth of 0.12 nm. Inthis exemplary implementation, the center wavelength of the filter wasfixed at 1290 nm. The detector signal can generate short pulses when theoutput wavelength of the laser is swept through the narrow passband ofthe fixed-wavelength filter. The timing of the sync pulse may becontrolled by changing the center wavelength of the filter.

FIG. 4A shows a graph of exemplary first output characteristics (laserspectrum vs. wavelength) of the laser source according to the presentinvention, and FIG. 4B is a graph of exemplary second outputcharacteristics (output power vs. time) of the laser source according tothe present invention. Turning to FIG. 4A, curve 110 represents theoutput spectrum of the laser measured by the optical spectrum analyzerin peak-hold mode, e.g., when the polygon arrangement spins at 15.7 kHz.The edge-to-edge sweep range was observed to be from 1282 nm to 1355 nm,equal to the free-spectral range of the filter. The Gaussian-likeprofile of the measured spectrum, rather than a square profile, can bemainly due to the polarization-dependent cavity loss caused bypolarization sensitivity of the filter and the birefringence in thecavity. It may be preferable to adjust the polarization controllers toobtain the maximum sweep range and output power. In FIG. 4B, curve 114is the output of the exemplary laser in the time domain. The upper trace112 is the sync signal which may be obtained through thefixed-wavelength filter. The amplitude of power variation from facet tofacet was less than 3.5%. The peak and average output power was 9 mW and6 mW, respectively. The y-axis scale of the curve 110 of FIG. 4A can becalibrated from the time-domain measurement, because the opticalspectrum analyzer records a time-averaged spectrum due to the lasertuning speed being much faster than the sweep speed of the spectrumanalyzer.

A frequency downshift in the optical spectrum of the intra-cavity laserlight may arise as the light passes through the SOA gain medium, as aresult of an intraband four-wave mixing phenomenon. In the presence ofthe frequency downshift, greater output power can be generated byoperating the wavelength scanning filter in the positive wavelengthsweep direction. FIG. 5 shows an exemplary illustration of a normalizedpeak power of the laser output measured as a function of the tuningspeed. The negative tuning speed can be obtained by flipping theposition of the collimator and the orientation of the grating withrespect to the optic axis 38 of the exemplary embodiment of thearrangement according to the present invention. It is preferable to makethe physical parameters of the filter identical in both tuningdirections. The result shows that the combined action of self-frequencyshift and positive tuning allows higher output to be obtained andenables the laser to be operated at higher tuning speed as isdemonstrated in the curve 120. Therefore, the positive wavelength scanmay be the preferable operation. The output power can be decreased withan increasing tuning speed. A short cavity length may be desired toreduce the sensitivity of the output power to the tuning speed. In suchcase, a free-space laser cavity may be preferred.

An exemplary embodiment of a free-space extended-cavity semiconductortunable laser arrangement according to the present invention is depictedin FIG. 6. A semiconductor waveguide 162 made on a substrate chip 160can be coupled to the polygon scanning filter via a collimating lens180. The front facet 164 thereof may be anti-reflection coated, and theoutput facet 166 may be cleaved or preferably coated with dielectrics tohave an optimal reflectivity. The laser output 190 may be obtainedthrough the output coupling lens 182. The sync output may be taken byusing a lens 140, a pinhole 142, and a photodetector 144 positioned onthe 0-th order diffraction path for the light which is onretro-reflection from the polygon scanner 24. The photodetector 144 cangenerate a short pulse when the focus of the optical beam of aparticular wavelength sweeps through the pinhole 142. Other types ofgain medium include but are not limited to rare-earth-ion doped fiber,Ti:Al₂O₃, and Cr³⁺:forsterite. The first and second lenses 20, 22 can bepreferably achromats with low aberration particularly in field curvatureand coma. The collimating lenses 180, 182 are preferably asphericlenses.

FIG. 7 shows another exemplary embodiment of the wavelength tunablefilter which includes an input collimating lens 12, diffraction grating16, focusing lens 200, and a spinning disk 210, as shown in FIG. 7. Thediffraction grating 16 preferably has a concave curvature that has afocal length and may thus eliminate the need for the use of the focusinglens 200. The diffraction grating may be replaced by other angulardispersive elements such as a prism. Preferably more than one reflector212 can be deposited on the surface of the spinning disk 210.Preferably, the reflectors 212 may include multiple narrow stripesperiodically and radially patterned. The material for the reflectors ispreferably gold. The disk 210 can be made of a lightweight plastic orsilicon substrate. Instead of the reflectors deposited on the topsurface of the disk, the disk can have a series of through holesfollowed by a single reflector attached to the back surface of the diskor supported independently from the disk. Incident from the opticalfiber 10, the optical beams of different wavelengths are illuminated onthe surface of the disk into a line after being diffracted by thegrating 16 and focused by the lens 200. The beam that hits thereflectors of the spinning disk may be retro-reflected and received bythe optical fiber 10. For example, a mirror 202 may be used tofacilitate the access of the beam onto the disk.

The distance from the lens 200 to the reflectors of the disk 210 may beapproximately equal to the focal length, F, of the lens 200. The tuningrange of the filter may be given by Δλ=p cos β₀(D/F), where D denotesthe distance between the stripes. The width of the strip, w, canpreferably be substantially equal to the beam spot size, w_(s), at thesurface of the disk:

${w_{s} = {W{\frac{\cos\;\beta_{0}}{\cos\;\alpha} \cdot \frac{F/z}{\sqrt{1 + {f/z^{2}}}}}}},$

s where z=πw_(s) ²/2. Such formulation may lead to a FWHM filterbandwidth given by (δλ)_(FWHM)/λ₀=A·(p/m)cos α/W where A=√{square rootover (4ln2)}/π. For w>w_(s), the filter bandwidth may become greater,and for w<w_(s), the efficiency (reflectivity) of the filter can bedecreased by beam clipping. The orientation of the incident beam 30 withrespect to the optic axis of the lens 200 and the spinning direction 220may determine the sense of wavelength tuning. The positive wavelengthscan may be preferable, which is the case of the exemplary embodimentshown in FIG. 7.

Two exemplary applications of the exemplary embodiments of the presentinvention are described as follows. FIG. 8 shows a block diagram of anexemplary embodiment of the spectrally encoded confocal microscope(“SECM”) that uses the aforementioned tunable laser source 300. Thebasic principle of SECM has been described in detail in U.S. Pat. No.6,341,036, the disclosure of which is incorporated herein by referencein its entirety. An exemplary probe 310 includes a transmission grating312 provided between two silicon prisms 314, 316, a collimator 318, anda microscope objective lens 320. The probe is equipped with a microactuator 322 to scan the beam onto a different location of the sample330. The actuator 322 may be driven by an actuator driver 324 atsubstantially slower speed than the tuning speed of the laser source.The probe motion is preferably rotary or translational and issynchronized to the sync output of the laser source. In one example, thewavelength sweep frequency may be 15.7 kHz, and the probe scan frequencycan be 30 Hz, which allows 30 frames of image to be obtained in 1second. The objective lens 320 has a nigh numerical aperture to providea transverse resolution of an order of micrometers and a confocalparameter of a few micrometers. The focus of the optical beam may becontinuously scanned in time over the sample 330 by the swept outputwavelength of the optical source and the scanning motion of the probe.The optical power returned from the sample is proportional to thereflectivity of the sample within a small section where the beam wasfocused down to a narrow waist at a given time. Two dimensional en-faceimage of the sample is constructed by a signal processor 344. Thedetector 340 is preferably an avalanche photodiode (“APD”) followed by atransimpedance amplifier 342. The reflected power may be receivedthrough a Faraday circulator 350 or a fiber-optic coupler.

Another exemplary application of the exemplary embodiments of thepresent invention is for optical coherence tomography (“OCT”) thedetails of which are described in U.S. Pat. No. 5,956,355, thedisclosure of which is incorporated herein by reference in its entirety.In one exemplary configuration, depicted in FIG. 9, an output of atunable source 300 may be directed to a sample 330 through a fiber-opticcoupler 410. An objective lens 412 in the probe may typically provide afocus near the surface or within the sample 330. The reference mirror420 can be placed in a reference arm 120 at a position where an opticalpath length between two arms of the Michelson interferometer issubstantially matched. Alternatively, the reference path can beconfigured in a transmissive, non-reflective configuration. The detector430 may be a PIN photodiode followed by a transimpedance amplifier 432with finite frequency bandwidth. The detector may preferably incorporatepolarization diverse and dual balanced detection. The detector signalcan be processed in the processor 434 through a fast Fourier transformto construct the depth image of the sample. The probe may be scanned byan actuator 414 and an actuator driver 416 to allow a 3-dimensionalimage of the sample to be obtained.

FIGS. 10A and 10B show a top and perspective view of another exemplaryembodiment of the wavelength tunable filter according to the presentinvention. An angularly deflecting optical element 700 of this exemplaryembodiment can be a rotating polygon arrangement 24 where the facets ofthe polygon are on the inner diameter of a hollow cylinder. A dispersingelement 702 such as a diffraction grating can be placed at the center ofthe polygon arrangement 24. Light can be delivered to the gratingthrough an optical fiber and collimated onto the grating so that eachfrequency component of the light is diffracted through a different angle(Θ). Only one narrow range of frequencies may be substantiallyorthogonal to one facet of the polygon arrangement 24, and thereforesuch frequency range may be reflected back to the diffraction gratingand collected by the optical fiber 704/706. When the cylinder rotates, asurface normal direction for the illuminated polygon arrangement's facetmay align with a new narrow frequency range. By rotating the cylinder,frequency tuning can thereby be achieved. When the cylinder rotationangle becomes large, an adjacent facet of the polygon arrangement 24 canbecome aligned with the light diffracted from the grating and the filterwill repeat another frequency tuning cycle. The free spectral range andfinesse can be controlled by appropriate choice of the polygon diameter,number of facets, collimated beam diameter and diffraction gratinggroove density.

The foregoing merely illustrates the principles of the invention.Various modifications and alterations to the described embodiments willbe apparent to those skilled in the art in view of the teachings herein.For example, the invention described herein is usable with the exemplarymethods, systems and apparatus described in U.S. Patent Application No.60/514,769. It will thus be appreciated that those skilled in the artwill be able to devise numerous systems, arrangements and methods which,although not explicitly shown or described herein, embody the principlesof the invention and are thus within the spirit and scope of the presentinvention.

What is claimed is:
 1. An apparatus comprising: an arrangementa lightsource configured to emit an electromagnetic radiation that has, thelight source including a cavity, a filter, and a gain medium which causea spectrum whoseof the electromagnetic radiation to have a meanfrequency that changes (i) at an absolute rate that is greater thanabout 100 terahertz per millisecond, and (ii) over a range that isgreater than about 10 terahertz.
 2. The apparatus according to claim 1,wherein the mean frequency changes repeatedly at a repetition rate thatis greater than 5 kilohertz.
 3. The apparatus according to claim 2,wherein the spectrum has a tuning range whose center is approximatelycentered at 1300 nm.
 4. The apparatus according to claim 2, wherein thespectrum has a tuning range whose center is approximately centered at850 nm.
 5. The apparatus according to claim 2, wherein the spectrum hasa tuning range whose center is approximately centered at 1700 nm.
 6. Theapparatus according to claim 2, wherein the spectrum has aninstantaneous line width that is smaller than 100 gigahertz.
 7. Theapparatus according to claim 1, the filter further comprising a polygonarrangement which is adapted to receive at least one signal that isassociated with the emitted electromagnetic radiation, and at least oneof reflect and deflect the at least one signal to a further location. 8.The apparatus according to claim 1, the light source further comprisinga laser resonating system forming an optical circuit and configured tocontrol a spatial mode of the electromagnetic radiation.
 9. Theapparatus according to claim 8, wherein the arrangement light sourcecauses the electromagnetic radiation to propagate substantiallyunidirectionally within at least one portion of the laser resonatingarrangement system.
 10. The apparatus according to claim 9, furthercomprising an optical circulator which is configured to control adirection of propagation of the electromagnetic radiation within thelaser resonating arrangement system.
 11. The apparatus according toclaim 8, further comprising an optical filtering system the filter beingconfigured to at least one of transmit or reflect at least one portionof the electromagnetic radiation based on a frequency of theelectromagnetic radiation, and wherein the at least one portion has afull-width-at-half-maximum frequency distribution which is less thanabout 100 GHz.
 12. The apparatus of claim 1, wherein the filtercomprises a movable mirror.
 13. An apparatus comprising: a light sourcearrangement configured to emit an electromagnetic radiation, the lightsource arrangement including a cavity and a gain medium; and a filterintegrated with the light source arrangement, wherein the light sourcearrangement and the filter in combination produce an output that has aspectrum whose mean frequency changes (i) at an absolute rate that isgreater than about 100 terahertz per millisecond, and (ii) over a rangethat is greater than about 10 terahertz.
 14. The apparatus according toclaim 13, the filter further comprising a polygon arrangement which isadapted to receive at least one signal that is associated with theemitted electromagnetic radiation, and at least one of reflect anddeflect the at least one signal to a further location.
 15. The apparatusaccording to claim 13, the light source further comprising a laserresonating system forming an optical circuit and configured to control aspatial mode of the electromagnetic radiation.
 16. The apparatusaccording to claim 13, wherein the mean frequency changes repeatedly ata repetition rate that is greater than 5 kilohertz.
 17. An apparatuscomprising: a light source arrangement configured to emit anelectromagnetic radiation, the light source arrangement including acavity and a gain medium; and a filter combined with the light sourcearrangement, wherein the combination of the light source arrangement andthe filter produce an output that has a spectrum whose mean frequencychanges (i) at an absolute rate that is greater than about 100 terahertzper millisecond, and (ii) over a range that is greater than about 10terahertz.
 18. The apparatus according to claim 17, the filter furthercomprising a polygon arrangement which is adapted to receive at leastone signal that is associated with the emitted electromagneticradiation, and at least one of reflect and deflect the at least onesignal to a further location.
 19. The apparatus according to claim 17,the light source further comprising a laser resonating system forming anoptical circuit and configured to control a spatial mode of theelectromagnetic radiation.
 20. The apparatus according to claim 17,wherein the mean frequency changes repeatedly at a repetition rate thatis greater than 5 kilohertz.
 21. An apparatus comprising: a light sourcearrangement configured to emit an electromagnetic radiation; and afilter coupled to the light source arrangement, wherein the light sourcearrangement and the filter in combination produce an output that has aspectrum whose mean frequency changes (i) at an absolute rate that isgreater than about 100 terahertz per millisecond, and (ii) over a rangethat is greater than about 10 terahertz.
 22. The apparatus according toclaim 21, the filter further comprising a polygon arrangement which isadapted to receive at least one signal that is associated with theemitted electromagnetic radiation, and at least one of reflect anddeflect the at least one signal to a further location.
 23. The apparatusaccording to claim 21, the light source further comprising a laserresonating system forming an optical circuit and configured to control aspatial mode of the electromagnetic radiation.
 24. The apparatusaccording to claim 21, wherein the mean frequency changes repeatedly ata repetition rate that is greater than 5 kilohertz.
 25. An apparatuscomprising: a light source configured to emit an electromagneticradiation, the light source including a cavity, a filter including amovable reflector, and a gain medium which cause a spectrum of theelectromagnetic radiation to have a mean frequency that changes (i) atan absolute rate that is greater than about 100 terahertz permillisecond, and (ii) over a range that is greater than about 10terahertz.
 26. The apparatus according to claim 25, wherein the movablereflector comprises a movable mirror.
 27. The apparatus according toclaim 26, wherein the filter is tuned by varying a resonator lengthusing the movable mirror.
 28. The apparatus according to claim 25, thelight source further comprising a laser resonating system forming anoptical circuit and configured to control a spatial mode of theelectromagnetic radiation.
 29. The apparatus according to claim 28,wherein the light source causes the electromagnetic radiation topropagate substantially unidirectionally within at least one portion ofthe laser resonating system.
 30. The apparatus according to claim 29,further comprising an optical circulator which is configured to controla direction of propagation of the electromagnetic radiation within thelaser resonating system.
 31. The apparatus according to claim 28, thefilter being configured to at least one of transmit or reflect at leastone portion of the electromagnetic radiation based on a frequency of theelectromagnetic radiation, and wherein the at least one portion has afull-width-at-half-maximum frequency distribution which is less thanabout 100 GHz.